This invention relates to a method and apparatus for the determination of the viscoelasticity of liquids. The apparatus constitutes a rheometer that measures the complex [i.e. the (i) resistive and (ii) reactive components of the] mechanical impedance presented by a liquid to an oscillating surface of a transducer. The Viscoelastic properties of sample liquids are determined from the measured mechanical impedance.
The transducer and associated circuitry of the rheometer are of the general type disclosed in U.S. Pat. Nos. 3,710,614; 3,712,117; 3,762,429; 3,875,791, 4,299,119; 4,488,427; and 4,524,610. Also see the article entitled "Oscillation Viscometer", published by J. V. Fitzgerald and F. J. Matusik in the June 1986 issue of Measurements and Control.
While all liquids impede the motion of an oscillating surface, there is a large class of liquids from whose impedance their viscoelasticity can be determined. Polymeric liquids belong to this class.
The viscoelasticities (and other rheological properties) of polymeric liquids are important to the plastics industry. As a result, in recent years there has been great effort to develop instrumentation for rheological laboratory and inprocess measurement.
Instrumentation for the determination of the viscoelasticity of polymeric fluids in tanks and pipes has been diligently sought by operating engineers. It is well known that better uniformity in product could be more efficiently achieved if the viscous and elastic responses of the polymeric liquid could be adjusted during processing.
In process control two types of rheometers can be used, namely online and inline types, the inline type being preferred. In an online device a sample of liquid is diverted from a main pipe or vessel for measurement. In an inline device the measurement is made directly inside the pipe or vessel while the liquid is flowing, stirred, or stationary--i.e. without interrupting the process and without removing the liquid from its processing path.
A number of instruments have been developed for providing laboratory (and in a few instances, online) viscoelasticity data, as discussed below. However, there is need for an inline rheometer which measures viscoelasticity.